The relative importance of friendship to happiness increases with age

Happiness is predicted by social relationships in general and contact frequency in particular. This study aims to examine if the relative importance of social contacts with the closest family/relative, friend, and neighbor in happiness changes with advancing age. We used data for all participants aged 19 years and older (n = 229,099) in the 2019 Community Health Survey, which measured the frequency of contact with the closest relative/family, neighbor, and friend among a representative sample of Koreans between August 16 and October 31, 2019. The Shapley value decomposition method was used to measure the relative importance of each predictor of happiness. Overall, contact frequency was positively associated with happiness (p<0.001). The relative importance value of contact with the closest family, neighbor, and friend to happiness increased from 4.70%, 3.98%, and 7.35%, respectively, in the 19–29 years group to 8.09%, 4.44%, and 11.00%, respectively, in the 60 years and older group. Frequent interactions with the closest friend could have a greater impact on happiness in old age than those with the closest family and neighbor.


Introduction
There is growing evidence that social relationships are associated with happiness. How much one enjoys and derives satisfaction from close social relationships is a significant predictor of subjective well-being and life satisfaction [1]. People who value social relationships are more likely to be happy than those who value achievements in other life domains, such as education and career [2]. The strength of family and relationships with friends is related to happiness and life satisfaction, directly as well as indirectly through an impact on health [3]. Feeling connected socially has a positive influence not only on psychological well-being but also on physical well-being [4]. The positive impact of increasing interactions with friends and relatives on life satisfaction is so great that an effort was made to estimate a monetary value [5].
Researchers have long tried to explore why social relationships are important to happiness [6]. One hypothesis is that individuals have a strong need to belong, which may be satisfied by building stable and strong interpersonal relationships [7]. The quality of friendship plays an important role in happiness because quality friendship satisfies psychological needs to belong a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 connecting with friends and neighbors can contribute to the sense of independence for older adults [28].
Researchers explored the source of social support and its differential mechanism for enhancing the emotional well-being of the elderly. For example, spouse support is more important in decreasing negative affect, whereas friendship has a greater influence on increasing positive affect [29]. Friends have a greater impact on the morale of the elderly than their grandchildren [30]. Similarly, the quality of relationships with friends was more important for life satisfaction of older adults than that with their children [16].
In this context, this study was designed to investigate the predictors of happiness, with a focus on the frequency of contact with the closest family, neighbor, and friend. The research question of particular interest is whether the relative importance of the type of social contact changes across age groups. Having reviewed the literature on happiness as related to social relationships, we developed the following hypotheses.
1. The frequency of social contact with the closest family/relative, friend, and neighbor is positively associated with happiness.
2. The relative importance of the type of social contact to happiness changes across age groups.

Data
This secondary data analysis relied on data on contact frequency that were collected by the Korea Centers for Disease Control and Prevention in the Community Health Survey (CHS). The annual survey that began in 2008 collects data using in-person interviews. The CHS is administered to a representative sample of Korean adults aged 19 years and older. The 2019 CHS measured social relationships in one dimension of contact frequency, that is, the frequency of meeting or contacting with the closest relative (including the family member), neighbor, and friend. Contact frequency was measured by the question "How often do you see or contact with the closest relative/family, neighbor, and friend that you have contacted most often?" Response choices comprise 'less than once per month', 'once per month', 'two or three times per month', 'once a week', 'two or three times a week', and 'four or more times per week'. The CHS uses a complex survey design in which administrative areas were cluster-sampled using residential type as a stratification variable. A sample of households within each area were selected using systematic sampling. All members aged 19 and older of selected households were interviewed. All analyses were adjusted for sampling weights. The 2019 CHS surveyed a total of 229,099 adults aged 19 years and older between August 16 and October 31, 2019 [31]. All participants in the 2019 CHS were used for analysis.

Happiness measure
Happiness, which is often used interchangeably with subjective well-being, has been defined as consisting of three dimensions: global life satisfaction, presence of positive affect and absence of negative affect [32]. However, in the 2019 CHS, happiness was measured in one item question of "all things considered, how happy are you in life these days?" The respondent was asked to rate happiness on a scale of one to ten, where ten represents the happiest state and one the unhappiest state. We used the single item happiness score as the dependent variable in regression models.

Control variables
Previously, happiness is shown to be influenced by socio-demographic variables like sex, marital status, education level, living arrangement, employment status, and household income [33,34]. This present study examined the impact of contact frequency on happiness while controlling for the socio-demographic factors. Marital status was grouped into currently married, formerly married, and never-married. The formerly married group comprises divorced, widowed, and separated. Education level was grouped into up to elementary, middle, high school, and college and higher education. Living arrangement was dichotomized into living alone and cohabitating with others. Employment status was grouped into salaried, selfemployed, and economically inactive. Household income (in KRW 10,000) was log-transformed to account for the skewness of income data distribution. In addition to socio-economic variables, this study also controlled for subjective health status, which was measured on a fivepoint Likert scale (very healthy, healthy, neutral, unhealthy, and very unhealthy) using a single question in the 2019 CHS, "how would you assess your health in general?"

Statistical analysis
We first described the study population according to sociodemographic characteristics, subjective health status, and social contact variables. Then, we compared the mean happiness levels across the groups of different characteristics. We also estimated several ordinary least square (OLS) regression models for all ages as well as by age group. OLS has been implemented to estimate predictors of happiness and life satisfaction in the existing studies [34,35]. Happiness measured in the Cantril ladder scale has been treated as a theoretically equal interval and continuous measure of life satisfaction [35], justifying the use of ordinary least square models. The Cantril ladder scale employs a visual ladder-like tool with equal intervals drawn on the questionnaire. A similar visual tool was used to measure happiness in the Community Health Survey. One could argue that the ideal method would be to treat happiness as an ordinal scale variable, which would require implementing ordered probit models. However, the models produce pseudo R 2 , which was difficult to decompose by using the methods described below.
OLS produces R 2 , a measure of goodness-of-fit, which represents the total amount of variance in the dependent variable explained by all independent variables included in the model. The dependent variable in all regression models was a measure of happiness. The regression model for all ages was run with independent variables including sex, age, education level, marital status, living alone (yes or no), employment status (self-employed, salaried, economically inactive), log household income, subjective health status, number of contacts with the closest relative/family, the neighbor, and the friend. To test if age moderates the effect of contact frequency on happiness, we implemented a model with the interaction term, age*contact frequency. Regression models by age group omitted age as an independent variable.
To determine the relative importance of the predictors of happiness, we calculated the Shapley value by adopting the empirical strategy used by Lamu and Olsen (2016). While a common approach is to interpret coefficients as values representing the importance of predictors in a regression model, research on the complex concept of happiness is susceptible to correlations among predictors [1]. Therefore, we used the Shapley value method to decompose variations in regression models, which is a technique that has game theory at the foundation [36].
The Shapley value evaluates the marginal contribution of each predictor to happiness. The value is calculated for each predictor by comparing R 2 with or without the predictor in every possible OLS model. The sum of Shapley values for all predictors in a model is equal to the R 2 of the model. %R 2 measures the proportional contribution of each predictor to R 2 and sums to 100%. As a share of goodness-of-fit, %R 2 represents the relative importance value of each predictor of happiness. To determine how the marginal contribution of each predictor to happiness changes with advancing age, we divided respondents into four age groups: 19-29, 30-39, 40-59, and 60 years and older. Young adults under 30 years were categorized as a group as they may share socioeconomic characteristics in terms of schooling, employment, and marital status that are different from other age groups. A regression model was run for each age group. Shapley value and %R 2 with 95% confidence interval were computed using 1,000 bootstrap samples for each age group.
All statistical analyses were performed by using SAS version 9.4 (Cary, NC, USA) and R software 3.6.1 (R foundation, Vienna, Austria). The Institutional Review Board of Kongju National University approved the study protocol and waived the requirement for informed consent (reference No. KNU_IRB_2022-074).

Results
Of the study population, 50.4% were female (Table 1). 26.8% were aged 60 years and older. 42.0% attained college or higher education and 63.9% were currently married. 11.2% lived alone and 47.0% were salaried workers. 15.0% rated their general health to be either very unhealthy or healthy. 18.1%, 47.4%, and 16.7% of Korean adults did not have any contact per month with the closest relative/family, neighbor, and friend, respectively.
On average, men were happier than women (p<0.001, Table 2). The mean happiness level peaked in 30-39 years and increased with higher education, better subjective health, and more frequent contacts with the closest relative/family and the friend (p<0.001). The currently married and individuals that were cohabitating with others were happier than their counterparts.
In the models for all ages and the � 60 years group, having more frequent contacts with the closest relative/family was consistently and positively associated with happiness (Tables 3 and  4, p<0.001). The regression model with interaction terms showed that age and contact frequency had a positive interaction effect on happiness. Models for the younger groups (19-29, 30-39, and 40-59 years) show that people do not become happier by having more contacts with the closest relative/family (Table 4). However, having more contacts with the closest friend consistently increased happiness in all age groups. The relative importance value of contacting with the closest family, neighbor, and friend to happiness increased from 4.70%, 3.98%, and 7.35%, respectively, in the 19-29 years group to 8.09%, 4.44%, and 11.00%, respectively, in the 60 years and older group (Table 5).
Aside from contact frequency, subjective good health was positively associated with happiness in all regression models and had the greatest relative importance value of all predictors across all age groups. The value of subjective health status was greatest at 72.32% in [19][20][21][22][23][24][25][26][27][28][29] years and lowest at 45.64% in 40-59 years. Happiness was negatively associated with log household income in all regression models. However, the relative importance value of household income peaked in 19-29 years and thereafter decreased with advancing age, whereas that of employment status increased from 19-29 years to 40-59 years.

Discussion
Previous research emphasizes the role of social connections in happiness. Among Korean adults, satisfaction with family life was the most important predictor of subjective well-being [34]. Older Koreans, in particular, regarded being with family and good health as top two essential conditions for happiness in life [37]. To further examine the impact of various types of social contact on happiness, we specified regression models with contact frequency for various social relationships along with sociodemographic characteristics and subject health status as control variables, using publicly available nationwide survey data. Then, we applied the Shapley value decomposition method to measure the relative importance of different predictors of happiness by age group. This present study shows that contact with the closest relative/family is more important to happiness than that with either the closest neighbor or the closest friend among middle-aged people aged 30-59 years old. However, contact with the closest friend becomes more      important to happiness in young adults under 30 years and the 60 years and older group. This mirrors the finding that the relative importance value of the frequency of contact with the closest relative/family member increased from the 30-59 years to the 60 years and older group but only slightly, whereas the relative importance value of the frequency of contact with the closest friend has more than doubled from 30-59 years to 60+ years. Similar to the findings regarding the role of the closest family/relative is those of marital status. In our current analysis, marital status is a significant predictor of happiness in all age groups, which is consistent with the previous finding that the state of being married increases happiness [3]. However, the relative importance of marital status decreased by a considerable margin from the 30-59 years to the 60 years and older group. All these findings suggest that the roles of social contacts in happiness change in old age among adults after 30 years. The changing relationship between social contact and happiness with increasing age can be supported by the earlier finding that family and friends have differential effects on well-being [38]. Doing activities with friends and family increases positive affect in old age. However, unlike family activities, doing activities with friends also decreases negative affect [38]. According to a study among Americans aged 65 years and older, contact with friends would lead to fewer discussions about stressful experiences than contact with family members [39]. With old age comes the loss of previous roles. The negative feelings associated with loss can be mitigated by frequent interactions with the closest friend.
While our findings support the critical role of friendship in happiness, it should also be recognized that there could be cultural differences in the relative importance of social contacts. Among African Americans, contact with neighbors appears to be the only significant predictor of happiness among all social relationships examined [14]. Similarly, the frequency of contact with children and friends was not a significant predictor of life satisfaction among Canadian adults [16]. Efforts have been made to explain the variations from a social policy standpoint. The positive relationship between the frequency of contact with family and relatives appears to be stronger in countries with greater public social expenditures on family benefits and services; Korea spends the least on family benefits and services out of 22 countries studied [18]. In addition to highlighting the role of social contacts, this present study reveals the changing impact of economic indicators on happiness. While financial security could be important to happiness in old age, happiness was negatively associated with household income and its relative importance diminished with advancing age. The latter finding is consistent with the findings of a previous study based on a number of other developed countries [1]. In comparison, another measure of financial security, employment status, was shown to be more important to happiness than household income in adults aged 30 years and older. These findings suggest that not just economic well-being but social participation associated with employment promote psychological well-being and healthy aging. On this note, there is evidence that working in old age lowers mortality [40]. Of course, it is possible that happiness is a cause of active employment rather than an outcome. Happy people reported having less sick days according to a randomized controlled trial, strongly suggesting that happiness makes people healthier [41]. One plausible explanation for the relationship between happiness and health is that happiness has a positive impact on health through playing a mediating role in the autonomic nervous system and promoting healthy behaviors [42].
In the literature, there is abundant evidence on the positive impact of social relationships on happiness. However, little has been known as to whether the type of social interactions contributes differently to happiness across age groups. Against this background, this study revealed that the frequency of contact with the closest friend is more important to happiness than that of contact with the closest relative/family in old age. These findings, however, should be interpreted with caution due to the following limitations. First, this study relied on crosssectional data, which makes it problematic to draw causal inferences. Second, we used selfreported data, which may not accurately reflect the truth. Third, this study relied on a single measure of happiness and one dimension of social relationship, namely, contact frequency. This precluded a comprehensive examination of complex concepts of happiness and social relationship. Nonetheless, this study provides a rare glimpse into the changing relative importance of friendship to happiness as people age.

Conclusion
This paper examined the influence of contact frequency on happiness and estimated the relative importance values of type of social contacts in happiness, using data from a nationally representative sample of Koreans and a variance decomposition technique. This study found that the frequency of interacting with the closest relative/family, neighbor, and friend is important to happiness across all groups. Moreover, this study highlights the increasing importance of friendship in happiness in old age. For aging individuals to maintain or increase their friendship activities would be beneficial to their psychological well-being. These findings may inform social policy intended to promote successful aging in an aged society.